Pleated filter elements are known and used in many industrial fluid filtration systems. Pleated filters, such as those having a round cartridge configuration are commonly used in engine air cleaner applications. High efficiency particulate air filters (HEPA) and ultra low penetration air filters (ULPA) of various designs are available and used in atomic, chemical, pharmaceutical and computer industries. Health care facilities such as, operating rooms and laboratories also use HEPA filters to provide clean air environments. The rectangular flat panel type filter element is widely used in applications where HEPA or ULPA filter performance is necessary.
One problem associated with pleated filter elements is optimizing filter performance. An "ideal" pleated filter media should exhibit the following characteristics. One, perfectly straight "V" shaped pleats. This configuration provides the lowest resistance to fluid flow through the element and a uniform distribution of flow or fluid velocity across the entire surface of the filter. Two, highest pleat spacing (density) possible. Maximum density provides the greatest fluid flow for a given package size at the lowest practical pressure loss; or stated in another way, use of the least amount of filter media area (lowest raw material cost) to achieve the desired flow and pressure loss is desired. Three, in some applications, sufficient space between pleats to allow for dust cake buildup (i.e., dirt capacity) is desired. To date, the problem of forming and maintaining this "ideal" or optimum configuration both in the filter manufacturing process and during operation of the filter has yet to be resolved. Currently used filter media exhibit anomalies associated with poor filter construction quality and dynamic changes that occur when the forces generated by fluid flow distort the pleat shape from the "ideal".
The most common method used to improve performance of pleated filter media is to form continuous corrugations across the length of the pleat face, as is done in many types of engine filter designs. In most of these applications cellulose fibers or a combination of cellulose and synthetic fibers are chosen along with a resin binder to create this filter medium. These combinations are usually quite pliable or moldable. The typical practice is to use two cylindrical embossing rolls having a matched pattern extending entirely around the periphery of each and repeating continuously in a symmetrical arrangement across the roll face (length), feeding the flat filter medium between them to impart a contour that matches the corrugation pattern of the rolls. The intent is to create a structure that when pleated limits distortion of the "ideal" pleat shape by allowing contact only between tips of corrugations. However, this approach precludes the filter from reaching its greatest potential. Specifically, fluid flow through the filter media area covered by the contact of corrugation tips is essentially prevented by what is commonly referred to as "masking".
Continuous corrugation of filter media is generally limited to those materials made from substantially cellulose or synthetic fibers or combinations of them. Filter media incorporating a substantial degree of glass fiber, especially micro-fiberglass and combinations of glass and synthetic fibers in conjunction with very low amounts of binder resin such as found in HEPA, ULPA, and ASHRAE rated industrial air filtration applications generally cannot be continuously corrugated. These materials typically have a very low stiffness. It is, therefore, more difficult to fabricate and maintain the "ideal" pleat shape. These materials also exhibit a limited elongation or deformation potential with respect to cellulose based filter media. When run through a continuous corrugation roll process the media frequently is ruptured.
The common practice to create the "ideal" pleat shape for materials that are difficult to corrugate is to leave the entire surface of the sheet flat and insert or add a mechanical means to "space" apart or shape the pleat. These "spacers" often take the form of corrugated aluminum sheets between pleats, another method is the use of adhesive coated strings spaced apart and applied to the surface of the media. Still another method is to use polymeric extrusion of continuous beads spaced apart and allowed to harden to form a mechanical separator. As will be realized, a mechanical (impervious) device inserted within the pleat or between pleats will create a significant degree of media masking. Further, the inclusion of these devices results in additional cost of materials used to fabricate the filter element.
Most known materials suitable for these applications will rupture at the pleat tip if continuous corrugations are used. One recently available type of filter material is composed of substantially all glass fiber which can be embossed with an interrupted corrugation (commonly referred to as dimples). These dimples separate and hold pleats apart. Circular dimples, as well as elongated cigar shapes and continuous protrusions similar to corrugations seen commonly in cellulosic filter materials have been used. While filters using "self-spaced" media are known (see, e.g. U.S. Pat. No. 4,610,706 to Nesher; U.S. Pat. No. 2,945,559 to Buckman), significant limitations on filter life and efficiency due to bunching, bagging and deformation of filter pleats still exist.
Accordingly, there is a need for a "self-spaced" pleated filter element that optimizes filter performance by exhibiting minimal masking from airflow and dust (i.e. minimal media blockage due to adjacent pleats or adjacent sides of same pleat being pressed together) and providing a prestressed pleat face that reduces adjacent pleat face contact during pressure loading of the filter media.